The general characteristics of buoyant momentum jets in water are described. Previous analytical modelling techniques utilizing the entrainment concept for prediction of trajectory and residual physical properties are discussed, and an overview of the existing experimental data base is given. The limitations of previous analytical modelling techniques are enumerated, generally resulting from incomplete or inadequate equations of state. An existing comprehensive equation of state for pure and saline water is proposed for use in entrainment modelling. An original computerized procedure, based on appropriate conservation equations, is used to predict trajectory and physical properties of various buoyant momentum jets. Comparison is made with previous analytical and experimental results for the cases of quiescent, flowing, and stratified ambients. Finally, the comprehensive equation of state, coupled with the present computational procedure, is used to describe a complexly stratified ambient and the behavior of a buoyant momentum jet discharged into it.